Although current ventilation-perfusion lung scanning by radionuclide methods (Tc-99m-MAA and Xenon-133) is fairly effective for the diagnosis of pulmonary embolism, great controversy exists regarding "false positive" scans. This project is directed towards reducing the number of false positives to more acceptable levels and, hopefully towards reduction of the false negative rate as well. We will use transmission computed tomography for these purposes. Our pilot studies have demonstrated that lung distal to pulmonary arteries occulded by Swan-Ganz balloon catheters becomes visibily denser than surrounding normal lung when studied by C.T. The increased density varies from 50 to 200 Hounsfield units, depending upon position of the lesion and the interval which has elapsed between pulmonary arterial occulsion and C.T. In each case studied thus far, concurrent chest radiographs in three projections have been unremarkable with no observed alveolar infiltrate, pleural fluid, or atelectasis. Thus, C.T. has been able to detect subtle change after pulmonary vascular occlusion not identified by conventional radiography. This project will: (1) establish the time-course of the events under investigation, thereby determining the interval between vascular occlusion and successful identification by C.T.; (2) extend studies of pulmonary arterial occlusion from balloon catheter to experimental pulmonary embolus; (3) compare the relative abilities of standard radionuclide lung scanning and C.T.; (4) establish whether acute obstructive airway disease produces computed tomographic findings similar to or dissimilar to those already proven to occur after pulmonary arterial occlusion; (5) establish whether bronchial arterial flow is sufficiently increased in the post-occlusive lung to account for the increased density observed.